Proton exchange membrane fuel cells (PEMFC) are an attractive alternative for converting chemical energy to electrical energy. Given the expanding availability of hydrogen from alternative sources, PEMFC technology promises to reduce the dependence of the U.S. economy on foreign oil. Domestic primary fuel resources, i.e., natural gas, alcohols, and petroleum distillates, may be converted to hydrogen through steam reforming, water-gas shift, and partial oxidation processes. These secondary hydrogen sources are attractive, at least in the near term, as significant infrastructure exists within the U.S. for the processing and distribution of these primary fuels. Current PEMFC technologies are, however, extremely sensitive to trace levels of contaminants that exist within the aforementioned fuels, such as hydrogen sulfide, often found in natural gas, and carbon monoxide, a byproduct of all fuel-conversion processes.
Clearly, detection and removal of undesired contaminants in PEMFC fuel streams is a major technological challenge. Transition metals such as platinum and ruthenium make up the catalytically-active portion of the membrane in a PEMFC and the catalytic activity of these metals is susceptible to reduced functionality or permanent deactivation (poisoning) by carbon monoxide and sulfur. Cell voltage, current flux, and membrane life decrease dramatically at part-per-million levels (5-100 ppm) of carbon monoxide and PEMFC catalysts are even less tolerant to sulfur in the form of H2S or mercaptan. Considerable efforts are, therefore, underway to design more tolerant membrane structures, as well as advanced fuel pre-conditioners (located post-reformer) that can reduce carbon monoxide and sulfur concentrations to acceptable levels via reaction, separation, or a combination of both.
Chemical sensors play an important role in managing carbon monoxide and sulfur in a PEMFC unit by providing information on contaminant levels within the cell stack and fuel processors. In addition, feedback from chemical sensors is a necessary input to closed-loop control and process optimization strategies. While the need for carbon monoxide and sulfur detection within the PEMFC environment is well established, the availability of process-compatible devices is lacking. Sensing technologies based on wet or dry electrochemical cells are currently limited by choice of ionic conductor and lack of chemical specificity.
For example, detecting carbon monoxide in gases using solid state devices (dry cells) has matured to the point of commercialization (e.g., home carbon monoxide monitors). These types of sensors are based on supported metal-oxide semiconductors (SMOS). One key component to such devices is the availability of oxygen in the sensing environment to convert carbon monoxide to carbon dioxide. Essentially, oxygen atoms are removed from the surface via catalytic oxidation of carbon monoxide, which induces an oxygen ion gradient within the sensor that is detectible as a small electrical current passing through the ionic conductor. The concentration of oxygen in a reformats stream and/or fuel feed to a cell stack is quite limited thereby rendering SMOS sensors completely ineffective without elaborate sample preparation/handling schemes. Other known carbon monoxide sensor types involve a wet electrochemical cell and are equally challenging in a PEMFC system.
The development of carbon monoxide sensors for use in PEMFC systems is further complicated by the presence of other gases in the fuel stream that could potentially interfere with a reliable measurement. For example, incomplete steam reforming of methanol produces residual quantities of formic acid, formaldehyde, and methyl formate, in addition to un-reacted methanol. Imparting intrinsic chemical selectivity to SMOS sensors is currently an active field of research with incremental gains targeted towards niche applications. In principle, hydrogen, as well as any of the aforementioned compounds resulting from methanol reforming, can be oxidized by the sensor and thereby hinder an accurate determination of the carbon monoxide and/or sulfur content in the matrix. The same would be true for residual hydrocarbons resulting from incomplete processing of natural gas or petroleum distillate.
Another method for detecting carbon monoxide in PEMFC essentially involves creation of a mini-PEM and locating it within the sensing environment. There here are a number of difficulties associated with this technique, such as increased noble metal loading needed to attain the necessary sensitivity, given that the mini-PEM has a reduced surface area. In addition, oxygen must still be part of the sensing system if electrical measurements are to be obtained
Hydrogen sensors based on metal-insulator-semiconductor (MIS) technology have been in existence for more than two decades. Due to their extreme sensitivity to hydrogen, they have been used as leak detectors to assure safety in the work place. Currently, MIS sensors have only been used in limited applications. This is due to the fact that the surface chemistry by which these sensors operate can be strongly affected by parasitic reactions and deactivation by catalyst poisons which result in cross-sensitivities to analytes other than hydrogen and which may reduce or extinguish sensor performance. While the increased sensitivity of these sensors is undesirable for industrial process monitors, this sensitivity renders them well suited for monitoring fouling agents in the PEMFC environment. MIS sensors with catalytic metal gates of palladium (Pd) are devices with proven capabilities to detect sub-ppm quantities of hydrogen in relatively inert atmospheres. It is well known that Pd-MIS structures, such as capacitors, field effect transistors, and tunneling diodes, saturate at low partial pressures of hydrogen (less than 1% at 1 atmosphere), and therefore can not function as a hydrogen sensor in a PEMFC system. The use of such Pd-MIS sensors in the detection of carbon monoxide is, however, unknown.